Really depends on how much coin you want to drop.
The turbo is a great investment, it doesnt give you much power though. What it does do is is provide more air in the form of boost so that you can turn your pump up for more fuel (the fuel is what gives you the power), and lets you run longer WOT without EGT's getting crazy high.
The first thing to do is make sure that its running good. Good injectors, IP, proper timing, and all the fuel lines in good shape is a good investment. Unless your going for crazy high horsepower dont get fooled into DPS injectors, they grind the tip off so they flow a lot of fuel but it doesnt atomize so fuel economy goes way down. High performance pumps are out there, they are a little on the rare side though. They can mess with the internals to make them flow more and what not. I would stick with regualar BB or G code injectors (Regular N/A and turbo IDI injectors, respectivly).
Then we can get into a little more power. Take the pistons and have them shaved down to reduce compression some. You cant go to much though, I forget the spec on how much you can go without them getting to thin. That'll reduce the compression ratio so you can run more boost. You would need to do a custom turbo setup, either mess with the housings on a turbo designed for an IDI, or a twin turbo setup (I like sequential turbo's, like what the 6.4 PSD uses), or something along those lines. Of course while shaving the pistons I would balance all the internals and have everything gone through. Clean the heads and exhaust manifolds up, and use ARP studs everywhere (heads, intake manifold, exhaust manifold, ect...ect...). Then you would need a custom pump, find an old sled puller and see what they know, back in the day they knew a lot of things they could do to squeeze every ounce of fuel they possibly could out of a Stanadyne DB2 pump. Run the timing more advanced. I would shy away from injectables, I just dont like them (like water/meth, propane, nos). Regrind the cam.
Basically its all dependent on how much you wanna spend.
Yup, that is what the rumor is, first diesel into the 9's. They had to use rubber freeze plugs because if they used normal ones they would bend because the block would actually flex when pulling. All back in the day before P-pumped cummins, electronics, ball bearing turbo's, ect